1. Field of the Invention
The present invention is generally related to laser eye surgery systems, devices, and methods. In particular, the present invention provides an adjustable focus patient fixation system which can accommodate refractive errors in a patient""s eye, presenting a target which appears to the eye to be in focus, and thereby enhancing the patient""s ability to stabilize the eye by viewing the target. In some embodiments, the present invention allows patients to maintain accurate focus and enhanced stabilization on a viewing target during changes in the refractive characteristics of the eye by coordinating focus adjustments of the target system with a photorefractive therapy. Hence, the present invention is particularly useful for enhancing the accuracy and efficacy of laser eye surgical procedures such as photorefractive keratectomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), and the like.
Selective photoablation of corneal tissues benefits from precise alignment between the eye and a therapeutic laser beam. Known laser eye procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye to alter its refractive power. The laser removes a selected portion of the corneal tissue, often to correct refractive errors of the eye. Ultraviolet laser ablation results in photodecomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photochemically, directly breaking the intermolecular bonds.
Laser ablation procedures can remove the targeted stroma of the cornea to change the cornea""s contour for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and the like. Control over the distribution of ablation energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, fixed and moveable apertures, controlled scanning systems, eye movement tracking mechanisms, and the like. These laser eye surgery systems are adapted for use while the patient is awake. The position of the patient""s head will often be stabilized using a headrest pillow, a chin rest, a bite bar, or the like. The patient can further enhance alignment between the eye and the therapeutic laser beam by focussing on a target during the procedure.
Known visual fixation targets typically include a light emitting diode (LED) which is optically positioned about xc2xc meter in front of the patient. Unfortunately, patients do not wear corrective lenses during photorefractive surgery. As a result, the target may be blurred or out of focus for many patients. Even more problematic, the optical characteristics of the patient""s eye may change significantly during laser eye surgery. As a result, the patient""s ability to hold her eye steady by viewing the target can be compromised. In fact, patients have reported losing site of the blurry targets during laser eye surgery. This may cause the patient to look away from the target, degrading alignment between the laser eye system and the eye, and thereby decreasing the accuracy and efficacy of the refractive therapy.
In light of the above, it would be desirable to provide improved ophthalmological systems, devices, and methods. It would be particularly desirable to provide enhanced techniques for stabilizing an eye having a significant refractive error throughout a laser eye surgery procedure. It would further be desirable to provide enhanced methods and devices for initially establishing and maintaining alignment with a patient fixation system to provide enhanced eye stabilization before and during laser eye surgery and other therapeutic or diagnostic procedures for the eye.
2. Description of the Background Art
U.S. Pat. No. 4,478,499, describes an operation microscope which incorporates an eye fixation device. U.S. Pat. No. 5,549,597, describes an in situ axis alignment module for determining the astigmatic axis of a patient, and for aligning the cylindrical axis of a laser ablation system for ophthalmological surgery.
U.S. Pat. No. 5,258,787, describes an ophthalmologic apparatus having an illumination optical system for directing light onto a prescribed point of an eye, and an observation optical system for observing an image of the prescribed point. U.S. Pat. No. 5,557,352, describes a method and apparatus for measuring the visual acuity and refraction of the human eye during and immediately after ocular surgery.
The present invention generally provides improved laser eye surgery devices, systems, and methods. The invention generally enhances the alignment between the eye and a laser beam of a laser eye surgery system using a visual fixation system having an optical train. The optical train of the fixation system allows an eye having a significant refractive error to be accurately focused at a fixation target. To accommodate the refractive error, the optical train will often project an image of the target so that the image is selectively focussed in front of, at, or behind the plane of the patient""s eye. The present invention also encompasses the calculation of the proper projection distance to accommodate the refractive error of the eye, the calculation preferably based at least in-part on the eye glass prescription for that eye.
A particular advantage of the present invention is that it allows the patient to focus upon (and minimize misalignment with) the target system while the eye undergoes significant refractive changes. For example, a patient undergoing a photorefractive therapy for 4.0 D hyperopia will have a significant change in the refractive configuration of the eye during the therapy. To maintain alignment between the changing eye and the laser beam, the present invention encompasses dynamically varying the image plane of the projected target image. Initially, the target image will be projected posterior to the plane of the hyperopic eye. Gradually, as the refractive configuration of the eye is corrected, the projected target image can be moved away from the, plane of the patient""s eye. Ideally, a computer controller dynamically varies the position of the projected target image in coordination with the photorefractive therapy. Where the photorefractive therapy proceeds in incremental steps, the adjustable optical train of the fixation system may also be incrementally adjusted, for example, by rotating a turret to select an alternative lens of the optical train. As the adjustable optical train helps the patient focus on the fixation target, the invention greatly enhances the patient""s ability to stabilize the eye as it undergoes these changes, and thereby enhances the accuracy and efficacy of the laser resculpting process.
In a first aspect, the present invention provides a laser eye surgery method. The method comprises projecting a target at a first distance from the eye. This projection allows the target to appear in focus to the eye. The eye is stabilized by viewing the-target through a cornea of the eye. Refraction of the stabilized eye is altered by selectively removing a portion of the cornea. The target is projected toward the altered eye at a second distance from the eye so that the target appears in focus to the altered eye.
In many embodiments, a target optical train will be adjusted to move a projected image of the target from the first distance to the second distance. For example, when the refractive therapy comprises a treatment for hyperopia, the image can initially be disposed posterior of the cornea. In contrast, when the refraction is altered so as to decrease myopia, the image will first be disposed anterior of the cornea. The image will typically move farther from the eye as the refractive error is corrected. For example, during laser in situ keratomileusis (LASIK), the distance between the projected image and the plane of the eye increases during the therapy.
In other embodiments, particularly where the refraction altering step is performed as a series of incremental improvements to a refractive error of the eye, the target image will be adjusted in a series of corresponding incremental changes. Alternatively, an appropriately designed Fresnel zone plate can simultaneously produce multiple target images in front of and/or behind the patient""s eye. Another approach would be to use a holographic optical element such as an appropriately designed diffractive optic hologram or Fresnel zone plate to project a pre-selected target shape at multiple desired positions in front of and/or behind the eye. The projected target shapes may have differing colors (or the like) at differing positions. Alternatively, a continuum of target images might be projected so as to define a line along the optical axis of the eye. The patient will then see the target image at the portion of the line corresponding to the proper separation distance between the eye and the target image location.
Preferably, the first distance will be calculated based at least in-part on an eye glass prescription of the eye. At least one intermediate distance may also be calculated based on an intermediate refractive configuration of the eye. This intermediate configuration may be either measured or estimated based on the known photoablative effects of a laser beam during photorefractive therapy, for example. A processor may simultaneously adjust the optical train and control photoablative sculpting of the cornea so that the target remains substantially in focus to the eye throughout the procedure.
In another aspect, the present invention provides a method comprising projecting a target toward an eye having a refractive error. The target is projected so that a focused image of the target is separated from the eye by a pre-calculated separation distance, and so that the target appears to be in focus to the eye. The eye is stabilized by viewing the projected target through a cornea of the eye.
In another aspect, the present invention provides a laser eye surgery system for resculpting a cornea of a patient""s eye. The laser system comprises a laser to produce a laser beam for photoablating a portion of the cornea. Delivery optics are optically coupled to the laser so as to effect a predetermined change in refraction of the cornea with the laser beam. A target optical train is aligned with the delivery optics to help the patient stabilize the eye. The optical train is capable of projecting the target toward the eye at a plurality of separation distances from the eye. This allows the system to accommodate a plurality of corneal refractive configurations of the eye.
Preferably, a computer controller couples the laser system delivery optics to the target optical train so that the optical train moves an image of the target in coordination with changes in the cornea. This allows the image to appear in focus to the eye despite the refractive changes taking place. The movement of the image may be smooth and gradual, or may be incremental. Preferably, at least a portion of the delivery optics and target fixation system are coaxially aligned. In some embodiments, the optical train may project the image simultaneously at a plurality of distances, typically using a holographic optical element, a Fresnel zone plate, or the like. It is generally preferable to project the target towards the eye so that the target appears within an angular sub-tense of less than about 80 mins. of arc at a pupil of the eye.
In yet another aspect, the present invention provides a laser eye surgery system for correcting refractive errors of an eye. The laser system comprises a laser to produce a laser beam for selectively removing a portion of a cornea of the eye. Delivery optics in an optical path of the laser beam transmit the laser beam toward the cornea so as to alter the eye from a first refractive configuration to a second refractive configuration. The target fixation system includes a target and target optics. The target optics are aligned with the optical path to help maintain alignment between the eye and the laser beam. The target optics project the target at a first distance and at a second distance. The target appears in focus to the eye when the cornea is in the first configuration and the target is projected at the first distance. The target also appears in focus to the eye when the cornea is in the second configuration and the target is projected at the second distance.